The Blue glaucus, or sea swallow, blue dragon, blue angel, blue sea slug, and blue ocean slug, (Glaucus atlanticus), is a species of small-sized blue sea slug, a pelagic aeolid nudibranch, a marine gastropodamollusk in the family Glaucidae. This is the only species in the genus Glaucus, but is closely related to glaucilla marginata, which is sometimes included in Glaucus.

This sea slugs feeds on other pelagic creatures including the venomous cnidarian, the portuguese man o' war. Because the sea slug stores stinging nematocysts from the cnidarian within its own tissues, a human picking up the sea slug may receive a very painful sting. For more information on the effects of the nematocysts.

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Characteristics

At maturity Blue Glaucus can be up to 3 cm in length. It is silvery grey on its dorsal side and dark and pale blue ventrally. It has dark blue stripes on its head. It has a tapering body which is flattened, and has six appendages which branch out into rayed, finger-like cerata.

The radula of this species bears serrated teeth.

Distribution and Habitat

This nudibranch is pelagic, and occurs throughout the world's oceans, in temperate and tropical waters. Regions where this slug is found include the East and South Coast of South Africa, European waters, the east coast of Australia and Mozambique. This species floats upside down on the surface tension of the ocean.

Blue Glaucus is able to feed on Physalia physalis due to its immunity to the venomous nematocysts. The slug consumes the entire organism and appears to select and store the most venomous nematocysts for its own use. The venom is collected in specialized sacs (cnidosacs), on the tip of the animal's cerata, the thin feather-like "fingers" on its body. Because blue Glaucus the venom, it can produce a more powerful and deadly sting than the Man o' War upon which it feeds.

With the aid of a gas-filled sac in its stomach, G. Atlanticus floats at the surface. Due to the location of the gas sac, the sea swallow floats upside down. The upper surface, actually the foot (the underside in other snails), has either a blue or blue-white coloration. The true dorsal surface (downwards in G. Atlanticus) is completely silver-grey. This coloration is an example of counter shading, which helps protect it from predators from below, sides, and above.

Almost all heterobranchs, Glaucus is a hermaphrodite, having both male and female reproductive organs. Unlike most nudibranchs, which mate with their right sides facing, sea swallows mate with ventral sides facing. After mating, both animals produce egg strings.